The most recent generation of electrophotographic printers and copiers employ charging members, such as a charge roller to perform the charging of the image forming member, such as a photoconductor (OPC) drum. Prior to the development of charge rollers, the OPC drum was charged using a corotron or a scorotron. Disadvantages of this older charging technology include the high voltages which must be generated to induce corona discharge, the charging inefficiency of corotron or scorotron drum charging devices, the ozone generated as a by-product of the corona discharge process, and the necessity of using discharge lamps to uniformly erase the OPC drum surface to eliminate the residual effects of the previous image.
Charge rollers offer improved performance over corotron or scorotron charging devices in each of these areas of deficiency. Because charge rollers operate in close proximity to the OPC drum, relative to corotron or scorotron devices, the voltages necessary to induce corona discharge are much lower than for corotron or scorotron devices. In addition, because charge rollers do not use a grid assembly to achieve charge uniformity on the OPC drum, they are much more efficient charging devices than corotrons or scorotrons. Furthermore, because charge rollers operate so that the corona discharge process occurs at lower voltages, the generation of ozone is substantially less than that of corotron or scorotron devices.
However, several quality and reliability problems, resulting from the use of charge rollers, have been recognized since charge rollers have experienced widespread use in electrophotographic cartridge products. To properly operate the charge roller it is necessary to locate it in close contact with the surface of the OPC drum. This is accomplished by applying a loading force to the charge roller which compresses it against the drum. This close contact allowed chemicals, released during or after the curing of the charge roller, to move onto the surface of the OPC drum. These solvent materials caused permanent degradation of the surface of the OPC drum. This problem resulted in a high rate of electrophotographic cartridge printing defects. The rate of occurrence of this problem was reduced by changes to the charge roller manufacturing process which reduced the amount of solvent used. However, the potential remains that with changes to the design or manufacturing process of the charge roller this problem would reappear.
Another problem which resulted from the use of charge rollers was also induced by the contact of the OPC drum and the charge roller. During shipping and handling of the electrophotographic cartridge, vibration and shock results in the charge roller moving over the surface of the OPC drum while it is in contact. This rubbing action induces charge in the outer charge transport layer of the OPC drum. The induced charge prevents the area which has experienced the rubbing action from discharging to the target voltage level during exposure of the OPC drum surface to form the latent electrostatic image. Many cycles of charge and discharge are required before the induced charge dissipates and the associated print defect is no longer visible. The resulting print defect can show up as light horizontal bands (for those printers which move the paper through the printer in the direction of the long dimension of the paper) across the page spaced by the circumference of the OPC drum. This problem continues to be a source of significant customer dissatisfaction requiring a solution.